Laser beam processing machine

ABSTRACT

A laser beam processing machine comprising a path distribution means for distributing a pulse laser beam oscillated by pulse laser beam oscillation means to a first path and a second path alternately, and one laser beam that passes through one of the paths and is converged by one condensing lens and the other laser beam that passes through the other path and is converged by the condensing lens are applied at different focusing points which have been displaced from each other in the direction of the optical axis, alternately with a time lag between them.

FIELD OF THE INVENTION

The present invention relates to a laser beam processing machine forapplying a pulse laser beam capable of passing through a workpiece toform a deteriorated layer in the inside of the workpiece.

DESCRIPTION OF THE PRIOR ART

In the production process of a semiconductor device, a plurality ofareas are sectioned by dividing lines called “streets” arranged in alattice pattern on the front surface of a wafer comprising a suitablesubstrate such as a silicon substrate, sapphire substrate, siliconcarbide substrate, lithium tantalite substrate, glass substrate orquartz substrate, and a circuit (function element) such as IC or LSI isformed in each of the sectioned areas. Individual semiconductor devicesare manufactured by cutting the wafer along the dividing lines to divideit into the areas each having a circuit formed thereon. To divide thewafer, there are proposed various methods making use of a laser beam.

U.S. Pat. No. 6,211,488 and Japanese Patent No. 3408805 disclose a waferdividing method comprising the steps of converging a pulse laser beam inan intermediate portion in the thickness direction of a wafer and movingthe pulse laser beam and the wafer relative to each other along dividinglines to form a deteriorated layer in the intermediate portion in thethickness direction of the wafer along the dividing lines, and exertingexternal force to the wafer to divide it along the deteriorated layers.

It is, however, conceivable not only that the deteriorated layer isformed in the intermediate portion in the thickness direction of thewater but also that the deteriorated layer is formed along the dividinglines in a portion from the back surface up to a predetermined depth orfrom the front surface to a predetermined depth in place of theintermediate portion in the thickness direction or in addition thereto.In either case, to divide the wafer along the dividing lines preciselyby exerting external force to the wafer, the thickness of thedeteriorated layer, that is, the size of the deteriorated layer in thethickness direction of the wafer must be made relatively large. Sincethe thickness of the deteriorated layer is 10 to 50 μm near the focusingpoint of a pulse laser beam, when the thickness of the deterioratedlayer is to be increased, the pulse laser beam and the wafer must bemoved relative to each other along each dividing line repeatedly bychanging the position of the focusing point of the pulse laser beam inthe thickness direction of the wafer. Therefore, in the case where thewafer is relatively thick, it takes long to form the deteriorated layerthick enough to divide the wafer precisely.

To solve the above problem, the applicant previously proposed a laserbeam processing machine, which was so constituted as to allow a pulselaser beam to converge at at least two focusing points that have beendisplaced from each other in the direction of its optical axis asJapanese Patent Application No. 2003-273341. With this laser beamprocessing machine, deteriorated layers can be formed at positions of atleast two focusing points, which are displaced from each other in thethickness direction of a workpiece, that is, a wafer at the same time.However, as this laser beam processing machine applies a laser beam withits focusing points displaced from each other on the same optical axisin the thickness direction of the wafer, a laser beam having a shallowfocusing point obstructs the application of a laser beam having a deepfocusing point, thereby making it impossible to form a desireddeteriorated layer.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a laser beamprocessing machine which has a constitution of a laser beam having adeep focusing point being not obstructed by the other laser beam havinga shallow focusing point, even when a laser beam is converged at twofocusing points which are displaced from each other on the same opticalaxis,.

To attain the above object, according to the present invention, there isprovided a laser beam processing machine comprising a chuck table forholding a workpiece and a laser beam application means for applying apulse laser beam capable of passing through the workpiece to theworkpiece held on the chuck table, the laser beam application meanscomprising a pulse laser beam oscillation means and atransmitting/converging means including an optical transmission meansfor transmitting a pulse laser beam oscillated by the pulse laser beamoscillation means and having one condensing lens for converging thepulse laser beam transmitted by the optical transmission means, wherein

the optical transmission means comprises a path distribution means fordistributing the pulse laser beam oscillated by the pulse laser beamoscillation means to a first path and a second path alternately, aplurality of mirrors and a beam splitter for putting together theoptical axis of a laser beam that has been distributed by the pathdistribution means and passes through the first path and the opticalaxis of a laser beam that has been distributed by the path distributionmeans and passes through the second path again and a focusing pointdepth displacing means that is arranged in any one path of the firstpath and the second path, and displaces the focusing point of one of thelaser beams, which have passed through the one path and is converged byone condensing lens, in the direction of the optical axis; and

one laser beam that passes through one of the paths and is converged bythe condensing lens and the other laser beam that passes through theother path and is converged by the condensing lens are applied atdifferent focusing points which have been displaced from each other inthe direction of the optical axis, alternately with a time lag betweenthem.

The above path distribution means has a polarization conversion meansfor dividing the pulse laser beam oscillated by the pulse laser beamoscillation means into vertically polarized light and horizontallypolarized light alternately and a beam splitter for distributing thevertically polarized laser beam and the horizontally polarized laserbeam obtained by the polarization conversion means to the first path andthe second path. The polarization conversion means comprises a modulatorfor dividing the pulse laser beam oscillated by the pulse laser beamoscillation means into horizontally polarized light and verticallypolarized light alternately and a pulse generator for providing a syncsignal for setting are petition frequency (f) to the pulse laser beamoscillation means and a sync signal having a frequency (f)/2 to themodulator. The modulator for dividing the pulse laser beam intohorizontally polarized light and vertically polarized light alternatelyis composed of a modulation element making use of an electro-opticeffect.

The above path distribution means comprises a modulator for distributingthe pulse laser beam oscillated by the pulse laser beam oscillationmeans to a first path and a second path alternately and a pulsegenerator for providing a sync signal for setting a repetition frequency(f) to the pulse laser beam oscillation means and a sync signal having afrequency (f)/2 to the modulator. The modulator for distributing thepulse laser beam to two paths alternately is composed of a modulationelement making use of an acoustic-optic effect.

The above focusing point depth displacing means changes the beamdivergent angle of the pulse laser beam.

Since the laser beams distributed to the first path and the second pathby the path distribution means are output alternately in the laser beamprocessing machine of the present invention, they are converged in theinside of the workpiece with a time lag between them. Therefore, evenwhen a laser beam passing through the first path and a laser beampassing through the second path are converged at focusing points whichare displaced from each other on the optical axis, the laser beam havinga deep focusing point is not obstructed by the other laser beam having ashallow focusing point.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic constitution diagram of a first embodiment of thelaser beam processing machine constituted according to the presentinvention;

FIG. 2 is a diagram showing a state where two deteriorated layers areformed in the inside of a workpiece by the laser beam processing machineof FIG. 1 at the same time;

FIG. 3 is a schematic constitution diagram of a second embodiment of thelaser beam processing machine constituted according to the presentinvention;

FIG. 4 is a schematic constitution diagram of a third embodiment of thelaser beam processing machine constituted according to the presentinvention;

FIG. 5 is a schematic constitution diagram of a fourth embodiment of thelaser beam processing machine constituted according to the presentinvention; and

FIG. 6 is a schematic constitution diagram showing another embodiment ofa focusing point depth displacing means provided in the laser beamprocessing machine constituted according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of a laser beam processing machine constitutedaccording to the present invention will be described in detail withreference to the accompanying drawings.

FIG. 1 is a schematic diagram of a first embodiment of a laser beamprocessing machine constituted according to the present invention. Theillustrated machine comprises a chuck table 3 for holding a wafer 2 as aworkpiece and a laser beam application means denoted by 4.

The chuck table 3 comprises an adsorption chuck 31 made from a porousmember or having a plurality of suction holes or grooves, and theadsorption chuck 31 is communicated with a suction means that is notshown. Therefore, a protective tape 21 affixed to the side of a surfaceon which a circuit is formed, of the wafer 2 as the workpiece is placedon the adsorption chuck 31, and the wafer 2 is suction-held on the chucktable 3 by activating the suction means that is not shown. The thusconstituted chuck table 3 is so constituted as to be moved in aprocessing-feed direction indicated by an arrow X in FIG. 1 by aprocessing-feed means that is not shown. Therefore, the chuck table 3and the laser beam application means 4 can move relative to each otherin the processing-feed direction indicated by the arrow X.

The laser beam application means 4 comprises a pulse laser beamoscillation means 5 and a transmitting/converging means 6 fortransmitting and converging a pulse laser beam oscillated by the pulselaser beam oscillation means 5. The pulse laser beam oscillation means 5oscillates a pulse laser beam 10 capable of passing through the wafer 2as the workpiece. As this pulse laser beam oscillation means 5 may beused a YVO4 pulse laser beam oscillator or YAG pulse laser oscillatorfor oscillating the pulse laser beam 10 having a wavelength of 1,064 nm,for example, when the wafer 2 is a wafer comprising a silicon substrate,silicon carbide substrate, lithium tantalite substrate, glass substrateor quartz substrate.

Description will be continued with reference to FIG. 1. Thetransmitting/converging means 6 of the laser beam application means 4 isinterposed between the pulse laser beam oscillation means 5 and thewafer 2 as the workpiece held on the chuck table 3. Thetransmitting/converging means 6 in the illustrated embodiment comprisesan optical transmission means 7 for transmitting the pulse laser beamoscillated by the pulse laser beam oscillation means 5 and a condensinglens 8 such as an objective lens for converging pulse laser beamstransmitted by the optical transmission means 7. The opticaltransmission means 7 comprises a path distribution means 71 fordistributing the pulse laser beam oscillated by the pulse laser beamoscillation means 5 to a first path 7 a and to a second path 7 balternately. This path distribution means 71 comprises a polarizationconversion means 711 for dividing the pulse laser beam oscillated by thepulse laser beam oscillation means 5 into vertically polarized light andhorizontally polarized light alternately, and a beam splitter 712 fordividing the vertically polarized beam and the horizontally polarizedbeam divided by the polarization conversion means 711 into the firstpath 7 a and the second path 7 b, respectively. The polarizationconversion means 711 comprises a modulator 711 a for dividing the pulselaser beam oscillated by the pulse laser beam oscillation means 5 intovertically polarized light and horizontally polarized light alternatelyand a pulse generator 711 b for providing a sync signal for setting arepetition frequency (f) to the pulse laser beam oscillation means 5 anda sync signal having a frequency (f)/2 to the modulator 711 a. As themodulator 711 a is used a modulation element making use of anelectrooptic effect in the illustrated embodiment.

The optical transmission means 7 in the illustrated embodiment comprisesa focusing point depth displacing means 73 arranged in the above firstpath 7 a, and a first mirror 74, a second mirror 75 and a beam splitter76 for aligning the optical axe of the vertically polarized laser beamand the optical axe of the horizontally polarized laser beam, which havebeen divided into the first path 7 a and the second path 7 b by theabove beam splitter 712, with each other again. The above focusing pointdepth displacing means 73 is composed of two convex lenses 731 and 732in the illustrated embodiment.

In the above-described laser beam processing machine, when a pulse laserbeam having a repetition frequency f (Hz) is to be oscillated from thepulse laser beam oscillation means 5, a sync signal for setting arepetition frequency (f) is provided from the pulse generator 711 b ofthe polarization conversion means 711 to control the repetitionfrequency (f) of the pulse laser beam oscillated by the pulse laser beamoscillation means 5. At the same time, the pulse generator 711 bprovides a sync signal having a frequency (f)/2 to the modulator 711 ain response to the signal, which serves as a trigger, provided to thepulse laser beam oscillation means 5. As a result, the pulse laser beam10 oscillated from the pulse laser beam oscillation means 5 is dividedinto vertically polarized light and horizontally polarized lightalternately when it passes through the modulator 711 a that is composedof a modulation element making use of an electro-optic effect (in thecase where polarization at the time of emitting a laser beam is linearlypolarized light). As for the division of the vertically polarized lightand the horizontally polarized light by the polarization conversionmeans 711, for example, two continuous pulses may be divided asvertically polarized light and the subsequent two continues pulses maybe divided as horizontally polarized light alternately.

The vertically polarized light and the horizontally polarized lightdivided by the modulator 711 a of the polarization conversion means 711alternately are distributed to a vertically polarized laser beam 10 aand a horizontally polarized laser beam 10 b by the beam splitter 712,respectively. That is, the vertically polarized laser beam 10 a passesthrough the beam splitter 712 to go straight to the first path 7 a whilethe horizontally polarized laser beam 10 b is reflected by the beamsplitter 712 to change its direction substantially at a right angle tothe second path 7 b. The vertically polarized laser beam 10 a goingstraight to the first path 7 a passes through the two convex lenses 731and 732 of the focusing point depth displacing means 73 to change itsbeam divergent angle. In the illustrated embodiment, its beam diametergradually increases as it becomes farther away from the convex lens 732,on a downstream side, constituting the focusing point depth displacingmeans 73. The vertically polarized laser beam 11 a whose beam divergentangle has been changed by passing through the focusing point depthdisplacing means 73 passes through the beam splitter 76 and is convergedat a focusing point Pa in the inside of the wafer 2 as the workpiece bythe objective condensing lens 8. Since the vertically polarized laserbeam 10 a going straight to the first path 7 a is converged by thecondensing lens 8 in a state where its beam diameter is graduallyincreased by passing through the focusing point depth displacing means73, its focusing point Pa is located at a position (i.e., lower positionin FIG. 1) deeper than the focusing point Pb which will be describedlater, of the horizontally polarized laser beam 10 a passing through thesecond path 7 b, that is, a position away from the condensing lens 8 inthe direction of the optical axis. The depth of this focusing point Pacan be suitably adjusted by moving the convex lens 731 or 732 as thefocusing point depth displacing means 73 in the direction of the opticalaxis.

Meanwhile, the horizontally polarized laser beam 10 b branched off tothe second path 7 b by the beam splitter 712 is reflected by the firstmirror 74, the second mirror 75 and the beam splitter 76 to change itsdirection substantially at a right angle, and its optical axis isaligned with the optical axis of the vertically polarized laser beam 10a passing through the above first path 7 a. The horizontally polarizedlaser beam 10 b whose optical axis has been thus aligned with theoptical axis of the vertically polarized laser beam 10 a is converged atthe focusing point Pb in the inside of the wafer 2 as the workpiece bythe condensing lens 8. As shown in FIG. 1, the focusing point Pb of thehorizontally polarized laser beam 10 b is located at a position (upperposition in FIG. 1) shallower than the focusing point Pa, that is, aposition close to the condensing lens 8 on the same optical axis as thatof the focusing point Pa of the above vertically polarized laser beam 10a.

Since the above vertically polarized laser beam 10 a and the abovehorizontally polarized laser beam 10 b are output alternately by themodulator 711 a of the above polarization conversion means 711, they areconverged in the inside of the wafer 2 as the workpiece with a time lagbetween their pulses. Therefore, the vertically polarized laser beam 10a and the horizontally polarized laser beam 10 b do not interfere witheach other, and the vertically polarized laser beam 10 a having a deepfocusing point is not obstructed by the horizontally polarized laserbeam 10 b having a shallow focusing point. As a result, deterioratedlayers W1 and W2 having desired thicknesses T1 and T2 can be formed nearthe focusing point Pa of the vertically polarized laser beam 10 a andthe focusing point Pb of the horizontally polarized laser beam 10 b,generally, in areas from the focusing point Pa and the focusing point Pbtoward an upward direction at the same time, respectively. Thedeteriorated layers formed in the wafer 2 as the workpiece are generallymolten and re-solidified (that is, molten when the vertically polarizedlaser beam 10 a and the horizontally polarized laser beam 10 b areconverged and then, re-solidified after the convergence of thevertically polarized laser beam 10 a and the horizontally polarizedlaser beam 10 b), namely, are in a state of voids or cracks, though thisdepends on the material of the wafer 2 or the intensities of theconverged vertically polarized laser beam 10 a and the convergedhorizontally polarized laser beam 10 b. The time lag between the timewhen the vertically polarized laser beam 10 a reaches the focusing pointPa and the time when the horizontally polarized laser beam 10 b reachesthe focusing point Pb is (1 second/repetition frequency). As for thevertically polarized laser beam 10 a and the horizontally polarizedlaser beam 10 b which are converged alternately, it is desired that thevertically polarized laser beam 10 a whose focusing point is away fromthe condensing lens 8 should be converged before the horizontallypolarized laser beam 10 b whose focusing point is close to thecondensing lens B.

The laser beam processing machine in the illustrated embodiment movesthe chuck table 3 (therefore, the water 2 as the workpiece held on thechuck table 3), for example, in a left direction in FIG. 1 while a pulselaser beam is applied as described above. As a result, two deterioratedlayers W1 and W2 having thicknesses T1 and T2 are formed along apredetermined dividing line in the inside of the wafer 2 at the sametime, as shown in FIG. 2. As described above, according to the laserbeam processing machine in the illustrated embodiment, the deterioratedlayers W1 and W2 having thicknesses T1 and T2 can be formed in two areaswhich are displaced from each other in the thickness direction of thewafer 2 as the workpiece, at the same time by using the single laserbeam application means 4. When the deteriorated layers W1 and W2 are tobe formed in the thickness direction continuously, the convex lens 731or 732 as the focusing point depth displacing means 73 is moved in thedirection of the optical axis, that is, in the vertical direction inFIG. 1 to shift the focusing point Pa of the vertically polarized laserbeam 10 a in an upward direction. The focusing point Pa is positionedlower than the focusing point Pb of the horizontally polarized laserbeam 10 b by the thickness T1.

The laser processing conditions are set as follows, for example.

-   -   Light source: LD excited Q switch Nd:YVO4 pulse laser    -   Wavelength: 1,064 nm    -   Pulse output: 2.5 μJ    -   Focusing spot diameter: 1 μm    -   Pulse width: 40 ns    -   Repetition frequency: 100 kHz    -   Processing-feed rate: 100 mm/sec

When the wafer 2 as the workpiece is thick and hence, the deterioratedlayers W1 and W2 having thicknesses T1 and T2 are not enough fordividing the wafer precisely along the dividing lines, the laser beamapplication means 4 and the chuck table 3 are moved relative to eachother by a predetermined distance in the direction of the optical axis,that is, the vertical direction indicated by the arrow Z in FIG. 1.Thereby, the focusing point Pa and the focusing point Pb are displacedfrom each other in the direction of the optical axis, that is, in thethickness direction of the wafer 2 as the workpiece, and the chuck table3 is moved in the processing-feed direction indicated by the arrow X inFIG. 1 while a pulse laser beam is applied from the laser beamapplication means 4. As a result, deteriorated layers W1 and W2 havingthicknesses T1 and T2 can be formed in the wafer 2 as the workpiece atpositions displaced in the thickness direction in addition to the abovedeteriorated layers W1 and W2.

A description is subsequently given of a second embodiment of the laserbeam application means 4 with reference to FIG. 3.

The laser beam application means 4 shown in FIG. 3 differs from thelaser beam application means 4 shown in FIG. 1 in that the focusingpoint depth displacing means 73 is arranged in the second path 7 b. Thisfocusing point depth displacing means 73 is composed of one convex lens733 and is interposed between the beam splitter 712 and the first mirror74. Since the constitution of the laser beam application means 4 shownin FIG. 3 is substantially the same as that of the laser beamapplication means 4 shown in FIG. 1 except for the focusing point depthdisplacing means 73, the same members are given the same referencesymbols and their descriptions are omitted.

In the laser beam application means 4 shown in FIG. 3, the verticallypolarized light and the horizontally polarized light divided by themodulator 711 a of the polarization conversion means 711 alternately areseparated into the vertically polarized laser beam 10 a and thehorizontally polarized laser beam 10 b by the beam splitter 712,respectively, like the laser beam application means 4 shown in FIG. 1.That is, the vertically polarized laser beam 10 a passes through thebeam splitter 712 to go straight to the first path 7 a while thehorizontally polarized laser beam 10 b is reflected by the beam splitter712 to change its direction substantially at a right angle to the secondpath 7 b. The vertically polarized laser beam 10 a going straight to thefirst path 7 a passes through the beam splitter 76 to be converged at afocusing point Pc in the inside of the wafer 2 as the workpiece by thecondensing lens 8. This focusing point Pc corresponds to the focusingpoint Pb of the horizontally polarized laser beam 10 b in theabove-described embodiment shown in FIG. 1.

Meanwhile, the horizontally polarized laser beam 10 b branched off tothe second path 7 b by the beam splitter 712 passes through the convexlens 733 as the focusing point depth displacing means 73 to change itsbeam divergent angle. In the illustrated embodiment, after thehorizontally polarized laser beam 10 b passes through the convex lens733, its divergent angle decreases so that its beam diameter graduallybecomes smaller as it becomes farther away from the convex lens 733. Thehorizontally polarized laser beam 10 b whose beam divergent angle hasbeen changed by passing through the convex lens 733 is reflected by thefirst mirror 74, the second mirror 75 and the beam splitter 76 at anglescorresponding to their installation angles, and its optical axis isaligned with the optical axis of the vertically polarized laser beam 10a passing through the first path 7 a. Since the beam divergent angle ofthe horizontally polarized laser beam 10 b incident on the condensinglens 8 has been changed by passing through the convex lens 733 as theabove focusing point depth displacing means 73, its beam diameter hasalso been changed. And, the horizontally polarized laser beam 10 bpassing through the condensing lens 8 is converged at a focusing pointPd in the inside of the wafer 2 as the workpiece. As the horizontallypolarized laser beam 10 b passing through the second path 7 b asdescribed above is converged by the condensing lens 8 in a state whereits beam diameter is gradually increased by passing through the focusingpoint depth displacing means 73, its focusing point Pd is located at aposition (lower position in FIG. 3) deeper than the focusing point Pc ofthe vertically polarized laser beam 10 a passing through the first path7 a, that is, at a position away from the condensing lens 8 in thedirection of the optical axis. The depth of the focusing point Pd can besuitably adjusted by moving the convex lens 733 as the focusing pointdepth displacing means 73 in the horizontal direction. Since thevertically polarized laser beam 10 a and the horizontally polarizedlaser beam 10 b are output alternately by the modulator 711 a of theabove polarization conversion means 711 also in the embodiment shown inFIG. 3, they are converged in the inside of the wafer 2 as the workpiecewith a time lag between them. Therefore, the vertically polarized laserbeam 10 a and the horizontally polarized laser beam 10 b do notinterfere with each other, and the horizontally polarized laser beam 10b having a deep focusing point is not obstructed by the verticallypolarized laser beam 10 a having a shallow focusing point. As a result,deteriorated layers W1 and W2 having desired thicknesses T1 and T2 canbe formed near the focusing point Pc of the vertically polarized laserbeam 10 a and near the focusing point Pd of the horizontally polarizedlaser beam 10 b substantially at the same time, respectively.

A description is subsequently given of a third embodiment of the laserbeam application means 4 with reference to FIG. 4.

The laser beam application means 4 shown in FIG. 4 differs from thelaser beam application means 4 shown in FIG. 1 in the path distributionmeans for distributing a pulse laser beam oscillated by the pulse laserbeam oscillation means 5 to the first path 7 a and the second path 7 balternately. That is, the path distribution means 72 in the embodimentshown in FIG. 4 is composed of a modulator 721 for dividing a pulselaser beam oscillated by the pulse laser beam oscillation means 5 intotwo different paths alternately and a pulse generator 722 for providinga sync signal for setting are petition frequency (f) to the pulse laserbeam oscillation means 5 and a sync signal having a frequency (f)/2 tothe modulator 721. As the modulator 721 is used a modulation elementmaking use of an acoustic-optic effect in the illustrated embodiment. Byproviding this path distribution means 72, the beam splitter 712 in thelaser beam application means 4 shown in FIG. 1 can be omitted. Since theconstitution of the laser beam application means 4 shown in FIG. 4 isidentical to that of the laser beam application means 4 shown in FIG. 1except for the path distribution means 72, the same members are giventhe same reference symbols and their descriptions are omitted.

In the laser beam application means 4 shown in FIG. 4, when a pulselaser beam having a repetition frequency f(Hz) is to be oscillated fromthe pulse laser beam oscillation means 5, a sync signal for setting arepetition frequency (f) is given from the pulse generator 722 tocontrol the repetition frequency (f) of the pulse laser beam oscillatedfrom the pulse laser beam oscillation means 5. At the same time, thepulse generator 722 provides, to the modulator 721, a sync signal havinga frequency (f)/2 in response to the signal, which serves as a trigger,provided to the pulse laser beam oscillation means 5. As a result, thepulse laser beams 10 oscillated from the pulse laser beam oscillationmeans 5 are divided into two different paths, that is, the first path 7a and the second path 7 b alternately when it passes through themodulator 721 composed of a modulation element making use of anacoustic-optic effect. For example, odd-numbered pulse laser beams aredivided into the first path 7 a and even-numbered pulse laser beams aredivided into the second path 7 b. As for the distribution of the pulselaser beams by the path distribution means 72, two continuous pulses maybe distributed to the first path 7 a and the subsequent two continuouspulses may be distributed to the second path 7 b alternately.

A first laser beam 10 c distributed to the first path 7 b by themodulator 721 of the path distribution means 72 passes through the twoconvex lenses 731 and 732 of the focusing point depth displacing means73 to change its beam divergent angle. In the illustrated embodiment,its beam diameter is so designed as to gradually increase as it becomesfarther away from the convex lens 732, on a downstream side,constituting the focusing point depth displacing means 73. The firstlaser beam 10 c whose beam divergent angle has been changed by passingthrough the focusing point depth displacing means 73 passes through thebeam splitter 76 and is converged at a focusing point Pe in the insideof the wafer 2 as the workpiece by the condensing lens 8. Since thefirst laser beam 10 c going straight to the first path 7 a is convergedby the condensing lens 8 in a state where its beam diameter is graduallyincreased by passing through the focusing point depth displacing means73, its focusing point Pe is located at a position (lower position inFIG. 1) deeper than the focusing point Pf of a second laser beam 10 dwhich will be described later and passes through the second path 7 b,that is, at a position away from the condensing lens 8 in the directionof the optical axis. The depth of the focusing point Pe can be suitablyadjusted by moving the convex lens 731 or 732 as the focusing pointdepth displacing means 73 in the direction of the optical axis.

Meanwhile, the second laser beam 10 d branched off to the second path 7b by the modulator 721 of the path distribution means 72 is reflected bythe first mirror 74, the second mirror 75 and the beam splitter 76 tochange its direction substantially at a right angle, and its opticalaxis is aligned with the optical axis of the first laser beam 10 cpassing through the first path 7 a. Thus, the second laser beam 10 dwhose optical axis has been aligned with the optical axis of the firstlaser beam 10 c is converged at a focusing point Pf in the inside of thewafer 2 as the workpiece by the condensing lens 8. As shown in FIG. 4,the focusing point Pf of the second laser beam 10 d is located at aposition (upper position in FIG. 1) shallower than the focusing pointPe, that is, a position close to the condensing lens 8 on the sameoptical axis as the focusing point Pe of the above first laser beam 10c.

Since the above first laser beam 10 c and the second laser beam 10 d areoutput alternately by the modulator 721 of the above path distributionmeans 72, they are converged in the inside of the wafer 2 as theworkpiece with a time lag between them. Therefore, the first laser beam10 c and the second laser beam 10 d do not interfere with each other,and the first laser beam 10 c having a deep focusing point is notobstructed by the second laser beam 10 d having a shallow focusingpoint. As a result, deteriorated layers W1 and W2 having desiredthicknesses T1 and T2 can be formed near the focusing point Pe of thefirst laser beam 10 c and near the focusing point Pf of the second laserbeam 10 d, respectively. The time lag between the time when the firstlaser beam 10 c reaches the focusing point Pe and the time when thesecond laser beam 10 d reaches the focusing point Pf is (1second/repetition frequency). As for the first laser beam 10 c and thesecond laser beam 10 d which are converged alternately, it is desiredthat the first laser beam 10 c whose focusing point is away from thecondensing lens 8 should be converged before the second laser beam 10 dwhose focusing point is close to the condensing lens 8.

A description is subsequently given of a fourth embodiment of the laserbeam application means 4 with reference to FIG. 5.

The laser beam application means 4 shown in FIG. 5 differs from thelaser beam application means 4 shown in FIG. 4 in that the focusingpoint depth displacing means 73 is arranged in the second path 7 b. Thisfocusing point depth displacing means 73 is composed of one convex lens733 and is interposed between the modulator 721 and the first mirror 74,like the focusing point depth displacing means 73 of the secondembodiment. Since the constitution of the laser beam application means 4shown in FIG. 5 is substantially identical to that of the laser beamapplication means 4 shown in FIG. 4 except for the focusing point depthdisplacing means 73, the same members are given the same referencesymbols and their descriptions are omitted.

In the laser beam application means 4 shown in FIG. 5, the first laserbeam 10 c divided by the modulator 721 goes straight to the first path 7a, passes through the beam splitter 76 and is converged at a focusingpoint Pg in the inside of the wafer 2 as the workpiece by the condensinglens 8, like the laser beam application means 4 shown in FIG. 4. Thisfocusing point Pg corresponds to the focusing point Pf of the secondlaser beam 10 d in the laser beam application means 4 shown in FIG. 4.

Meanwhile, the second laser beam 10 d branched off to the second path 7b by the modulator 721 of the path distribution means 72 passes throughthe convex lens 733 as the focusing point depth displacing means 73 tochange its beam divergent angle. In the illustrated embodiment, thedivergent angle of the second laser beam 10 d is gradually reduced bypassing through the convex lens 733 so that its diameter graduallybecomes smaller as it becomes farther away from the convex lens 733. Thesecond laser beam 10 d whose beam divergent angle has been changed bypassing through the convex lens 733 is reflected by the first mirror 74,the second mirror 75 and the beam splitter 76 at angles corresponding totheir installation angles, and its optical axis is aligned with theoptical axis of the first laser beam 10 c passing through the abovefirst path 7 a. Since the beam divergent angle of the second laser beam10 d incident on the condensing lens 8 has been changed by passingthrough the convex lens 733 as the focusing point depth displacing means73, its beam diameter has also been changed. The second laser beam 10 dpassing through the condensing lens 8 is converged at a focusing pointPh in the inside of the wafer 2 as the workpiece. Since the second laserbeam 10 d passing through the second path 7 b is converged by thecondensing lens 8 in a state where its beam diameter is graduallyincreased by passing through the focusing point depth displacing means73, its focusing point Ph is located at a position (lower position inFIG. 5) deeper than the focusing point Pg of the first laser beam 10 cpassing through the first path 7 a, that is, at a position away from thecondensing lens 8 in the direction of the optical axis. The depth of thefocusing point Ph can be suitably adjusted by moving the convex lens 733as the focusing point depth displacing means 73 in the horizontaldirection. Also in the embodiment shown in FIG. 5, as the first laserbeam 10 c and the second laser beam 10 d are output alternately by theabove modulator 721, they are converged in the inside of the wafer 2 asthe workpiece with a time lag between them. Therefore, the first laserbeam 10 c and the second laser beam 10 d do not interfere with eachother, and the second laser beam 10 d having a deep focusing point isnot obstructed by the first laser beam 10 c having a shallow focusingpoint. As a result, deteriorated layers W1 and W2 having desiredthicknesses T1 and T2 can be formed near the focusing point Pg of thefirst laser beam 10 c and near the focusing point Ph of the second laserbeam 10 d, respectively.

A description is subsequently given of another embodiment of thefocusing point depth displacing means 73 with reference to FIG. 6.

The focusing point depth displacing means 73 shown in FIG. 6 comprises aconvex lens 734 and a convex lens 735 which are spaced apart from eachother, and a first pair of mirrors 736 and a second pair of mirrors 737,which are interposed between the convex lens 734 and the convex lens735. The first pair of mirrors 736 consist of a first mirror 736 a and asecond mirror 736 b which are parallel to each other and fixed to amirror holding member (not shown) in a state where they keep spacingtherebetween. The second pair of mirrors 737 consist of a first mirror737 a and a second mirror 737 b which also are parallel to each otherand fixed on a mirror holding member (not shown) in a state where theykeep spacing therebetween. In the focusing point depth displacing means73 that is constituted as described above and shown in FIG. 6, when thehorizontally polarized laser beam 10 b or the second laser beam 10 dwhich has branched off to the second path 7 b, passes through the convexlens 734, the first mirror 736 a and the second mirror 736 b of thefirst pair of mirrors 736, the first mirror 737 a and the second mirror737 b of the second pair of mirrors 737, and the convex lens 735, thedivergent angle of the horizontally polarized laser beam 10 b or thesecond laser beam 10 d is so constituted as to increase so that its beamdiameter gradually becomes larger as it becomes farther away from theconvex lens 735. The divergent angle of the horizontally polarized laserbeam 10 b or the second pulse laser beam 10 d and its beam diameter whenit is incident on the above condensing lens 8, that is, the depthposition of the focusing point of the horizontally polarized laser beam10 b or the second pulse laser beam 10 d, which passes through thefocusing point depth displacing means 73 and is converged by the abovecondensing lens 8, can be suitably adjusted by changing the length ofthe optical path by varying the installation angles of the first pair ofmirrors 736 and the second pair of mirrors 737. For the adjustment ofthe installation angles, the mirror holding members (not shown) forholding the first pair of mirrors 736 and the second pair of mirrors 737are turned on a point Q where the first mirror 736 a and the firstmirror 737 a become point symmetrical to the second mirror 736 b and thesecond mirror 737 b, respectively. The above focusing point depthdisplacing means 73 may be interposed between the first mirror 74 andthe second mirror 75 in the above embodiments or may be arranged in thefirst path 7 a.

While the present invention has been described above based on theillustrated embodiments with reference to the accompanying drawings, itshould be noted that the present invention is in no way limited to theabove embodiments only but can be changed or modified in other variousways without departing from the scope of the present invention. Forexample, in the illustrated embodiments, the convex lens is used as thefocusing point depth displacing means. The lens as the focusing pointdepth displacing means may be a concave lens or a set of lenses.

1. A laser beam processing machine comprising a chuck table for holding a workpiece and a laser beam application means for applying a pulse laser beam capable of passing through the workpiece to the workpiece held on the chuck table, the laser beam application means comprising a pulse laser beam oscillation means and a transmitting/converging means including an optical transmission means for transmitting a pulse laser beam oscillated by the pulse laser beam oscillation means and having one condensing lens for converging the pulse laser beam transmitted by the optical transmission means, wherein the optical transmission means comprises a path distribution means for distributing the pulse laser beam oscillated by the pulse laser beam oscillation means to a first path and a second path alternately, a plurality of mirrors and a plurality of beam splitters for aligning an optical axis of a laser beam that has been distributed by the path distribution means and passes through the first path with an optical axis of a laser beam that has been distributed by the path distribution means and passes through the second path by the path distribution means again, and a focusing point depth displacing means that is arranged in any one of the first path and the second path and displaces the focusing point of one of the laser beams, which passes through one of the paths and is converged by the condensing lens, in the direction of the optical axis; and one laser beam that passes through one of the paths and is converged by one condensing lens and the other laser beam passing through the other path and is converged by the condensing lens are applied at different focusing points which have been displaced from each other in the direction of the optical axis, alternately with a time lag between them.
 2. The laser beam processing machine according to claim 1, wherein the path distribution means has polarization conversion means for dividing the pulse laser beam oscillated by the pulse laser beam oscillation means into vertically polarized light and horizontally polarized light alternately and a beam splitter for dividing a vertically polarized laser beam and a horizontally polarized laser beam divided by the polarization conversion means into the first path and the second path, respectively.
 3. The laser beam processing machine according to claim 2, wherein the polarization conversion means comprises a modulator for dividing the pulse laser beam oscillated by the pulse laser beam oscillation means into horizontally polarized light and vertically polarized light alternately and a pulse generator for providing a sync signal for setting a repetition frequency (f) to the pulse laser beam oscillation means and a sync signal having a frequency (f)/2 to the modulator.
 4. The laser beam processing machine according to claim 3, wherein the modulator for dividing the pulse laser beam into horizontally polarized light and vertically polarized light alternately is composed of a modulation element making use of an electro-optic effect.
 5. The laser beam processing machine according to claim 1, wherein the path distribution means comprises a modulator for dividing the pulse laser beam oscillated by the pulse laser beam oscillation means into a first path and a second path alternately and a pulse generator for providing a sync signal for setting a repetition frequency (f) to the pulse laser beam oscillation means and a sync signal having a frequency (f)/2 to the modulator.
 6. The laser beam processing machine according to claim 5, wherein the modulator for dividing the pulse laser beam into two paths alternately is composed of a modulation element making use of an acoustic-optic effect.
 7. The laser beam processing machine according to claim 1, wherein the focusing point depth displacing means changes the beam divergent angle of the pulse laser beam. 